Photolithography is used in manufacturing microelectronic devices to form precise patterns on a substrate surface. A photo mask or reticle generally is a square plate of transparent material, such as glass or fused silica. The photomask is initially created with the pattern to be transferred to the substrate, such as a semiconductor wafer. With subsequent processing steps, the patterned substrate surface forms the desired micro or nano scale electronic devices or features. A photo mask may be repeatedly used to imprint thousands of substrates. In use the back side may become contaminated by scratches, burrs or particles, which degrades the photo mask. Techniques for cleaning the back side have been proposed. However, while cleaning the back side, it is necessary to avoid affecting the patterned front side of the photo mask. This presents various engineering challenges. According, improved apparatus and methods are needed for photo mask back side cleaning.